The field of the present invention is screening systems and screening assemblies therefor.
Modern screening devices employed in commercial processing operations characteristically use tensioned screens which are subjected to either vibration or centrifugal action and frequently experience high loads and abrasive action. The requirements placed on the screen assemblies employed within such modern commercial devices long ago resulted in the abandonment of the familiar mounting methods used for such devices as window screens and the like. The tensioning loads place substantial structural requirements on the screen frames as do the vibrational or centrifugal loads and the weight and inertia of the material being processed therethrough. The loading and abrasive action also create specific problems at the intersection of the screen cloth and the screen frame. If these exceptional requirements are not specifically addressed, early screen failure is experienced, such failure manifesting itself as a slackening of the screen or parting of the screen from the frame, as frame warpage or failure, or as failure of the screen cloth at the intersection with the frame.
The association between the screen assembly and the overall screening device also requires attention in a commercial setting. For maximum use of any screening device, it is beneficial to insure both rigid placement and retention of the screen assembly therein and quick removal and replacement of that assembly. The large variety of tasks to which modern screening devices are put has required facile removal and replacement of screens for a number of reasons. In food processing or where multiple batch processing is employed, the screens must be removed for cleaning on a regular basis. Also in batch processing, the screens may require frequent replacement to vary the mesh size thereof. In processing certain materials, frequent cleaning is required to overcome the problem of screen blinding. And, of course, in highly abrasive environmehts, screens must be frequently replaced as they simply wear out at a rapid rate. Consequently, secure placement, quick removal and accurate positioning are essential requirements for screen assemblies in modern commercial screening devices.
As an example of a modern screening device, reference is made to U.S. Pat. No. 3,899,414 to Leslie T. Hansen entitled "Drilling Mud Separation System". This device includes a cylindrical housing employing a screen extending across the housing. The device is subjected to vibration for enhanced screening action. The disclosure of the Hansen patent is incorporated herein by reference as showing a typical screening device employed in a relatively harsh environment, that of well drilling mud processing.
In commercially designing screen assemblies, a variety of approaches have been used to meet the adverse conditions experienced and yet to attempt to keep the cost of such screens to a minimum. In some applications, the need to replace screens is so great that the cost of replacement screens becomes a major cost factor in the screening process. To this end, devices have been designed for facile replacement of the screen cloth itself in a relatively complicated frame which acts to tension the cloth when positioned. One such device is illustrated in U.S. Pat. No. 2,213,773 to L. G. Symons, entitled "Means For Supporting and Tensioning Screen Cloth". In spite of the advantageous feature that the screen frames themselves do not require replacement, such screen cloth replacement systems have not found general acceptance.
A variety of other efforts have been undertaken to reduce the cost of such screen assemblies by reducing the cost of the initial fabrication or by attempting to improve the life of the screen. One such successful screen assembly is illustrated in U.S. Pat. No. 3,915,775 to Calvin D. Davis entitled "Method of Bonding A Plastic Tension Ring To A Screen". The Davis device is generally of molded plastic with a central metal member for structural rigidity. Soft material is located at the intersection of the screen cloth with the screen frame to spread the stress loading of the screen at that point and reduce abrasion. The disclosure of the Davis patent is incorporated herein by reference as illustrative of such features. In spite of the utility and commercial success of this modern screen, the molding of the screen frame and the number of steps involved in the fabrication of the assembly makes such screen assemblies relatively expensive, particularly when highly abrasive environments force frequent replacement thereof.
In placing commercial screens in screening apparatus, a variety of approaches have been taken. One such approach is illustrated in the Hansen patent, U.S. Pat. No. 3,899,414, referred to above. In the Hansen device, the outer rim of the screen frame sits between two axially aligned portions of the cylindrical housing, all of which is held in place by a conventional clamp band. Another approach for mounting screen frames to allow for easy removal thereof is illustrated in U.S. Pat. No. 2,279,042 to D. J. Harrington, entitled "Screening Apparatus". In the Harrington device, cylinders and pistons are employed to hold down the corners of the screen frames. The complication of the Harrington device in a vibratory screening structure is highly disadvantageous. On the other hand, the Hansen device is uncomplicated and inexpensive but does require partial disassembly of the screening device for replacement of the screen. Caught between providing a very rigid supporting structure for the screen assemblies and providing an easily disassembled system, most modern commercial screening systems generally have exhibited either overcomplicated placement mechanisms or overcomplicated disassembly and assembly requirements.
Attempts have been made to employ elements attached to the structure of the screening system itself rather than an independent frame, for supporting the tension load of the screen. To this end, untensioned screens are generally placed in such devices and tensioned by means of inflatable members supported by extensions of the frame. In this regard, reference is made to U.S. Pat. No. 3,176,843 to R. J. Hoskins, et al., entitled "Screen Tensioner" and to French Pat. No. 883,576, issued Mar. 29, 1943. Each of these devices requires substantial structure associated with or added to the frame of the screening device to contain and control the expansion of the tensioning pneumatic member. It would appear from the disclosure of these devices that there would be no easy screen replacement, a difficulty encountered by the mechanisms employing other screen cloth replacement systems as well.